1. Field of the Invention
This invention relates to a bottom hole assembly comprising a rotary steerable directional drilling tool, which is useful when drilling boreholes into the earth.
2. Description of the Related Art
Rotary steerable drilling systems for drilling deviated boreholes into the earth may be generally classified as either “point-the-bit” systems or “push-the-bit” systems. In the point-the-bit system, the axis of rotation of the drill bit is deviated from the local axis of the bottom hole assembly (BHA) in the general direction of the new hole. The hole is propagated in accordance with the customary three point geometry defined by upper and lower stabilizer touch points and the drill bit. The angle of deviation of the drill bit axis coupled with a finite distance between the drill bit and lower stabilizer results in the non-collinear condition required for a curve to be generated. There are many ways in which this may be achieved including a fixed bend at a point in the BHA close to the lower stabilizer or a flexure of the drill bit drive shaft distributed between the upper and lower stabilizer. In its idealized form, the drill bit is not required to cut sideways because the bit axis is continually rotated in the direction of the curved hole. Examples of point-the-bit type rotary steerable systems, and how they operate are described in U.S. Patent Application Publication Nos. 2002/0011359; 2001/0052428 and U.S. Pat. Nos. 6,394,193; 6,364,034; 6,244,361; 6,158,529; 6,092,610; and 5,113,953 all herein incorporated by reference.
In the push-the-bit rotary steerable system there is usually no specially identified mechanism to deviate the bit axis from the local BHA axis; instead, the requisite non-collinear condition is achieved by causing either or both of the upper or lower stabilizers to apply an eccentric force or displacement in a direction that is preferentially orientated with respect to the direction of hole propagation. Again, there are many ways in which this may be achieved, including non-rotating (with respect to the hole) eccentric stabilizers (displacement based approaches) and eccentric actuators that apply force to the drill bit in the desired steering direction. Again, steering is achieved by creating non co-linearity between the drill bit and at least two other touch points. In its idealized form the drill bit is required to cut side ways in order to generate a curved hole. Examples of push-the-bit type rotary steerable systems, and how they operate are described in U.S. Pat. Nos. 5,265,682; 5,553,678; 5,803,185; 6,089,332; 5,695,015; 5,685,379; 5,706,905; 5,553,679; 5,673,763; 5,520,255; 5,603,385; 5,582,259; 5,778,992; 5,971,085 all herein incorporated by reference.
Although such distinctions between point-the-bit and push-the-bit are useful to broadly distinguish steering systems, a deeper analysis of their hole propagation properties leads one to recognize that facets of both are present in both types of deviated borehole steering systems. For example, a push-the-bit system will have a BHA that is not perfectly stiff, enabling the bit to be effectively pointed and so a proportion of hole curvature is due to the bit being pointed. Conversely, with point-the-bit systems that use a fixed bend offset, a change in hole curvature requires the bit to cut sideways until the new curvature is established. Changes in hole gauge and stabilizer wear effectively cause the bit to be pointed in a particular direction, which may or may not help the steering response, regardless of steering system type. In the extreme, push-the-bit systems that use drill bits with little or no side cutting ability may still achieve limited steering response by virtue of the aforementioned flexibility of the BHA or stabilizer/hole gauge effects.
It is into this broad classification of deviated borehole steering systems that the invention disclosed herein is launched. The hybrid steering system of the present invention breaks with the classical point-the-bit versus push-the-bit convention by incorporating both into a single scheme by design rather than circumstance.